343 
certain Monocotyledonous Leaves . 
remainder of the limb, except in the fact that they come to maturity 
precociously, while the rest of the leaf is still tiny and embryonic. Their 
early development seems to me a phenomenon of the same order as the 
very early development of the tendrils in the case of Smilax — a peculiarity 
also associated with the climbing habit. The forerunner tip is the most 
conspicuous feature of the Yam-leaf while it is quite young, but the rest of 
the limb soon catches it up, and then outstrips it, so that finally it sinks 
into entire insignificance. But such a reversal of the order of things is not 
a unique occurrence; it is, indeed, by no means unusual for the proportion 
of parts to alter completely in the course of ontogeny. In the embryonic 
leaf of Narcissus sp., for example, the small limb forms a mere appendage 
of the conspicuous sheath, but at maturity the relations are reversed, and 
the limb is the only obvious region, while the sheath, which originally 
exceeded it in size, now occupies only a minute proportion of the length of 
the leaf . 1 
Liliiflorae — Iridaceae. 
Since I have already dealt in this journal with the leaf-structure of the 
various tribes of the Iridaceae , 2 I will confine myself now to a single 
illustrative case. 
Tigridia. 
I have described, in the paper just cited, the origin of the highly 
complex leaf form of Tigridia Pavonia , Ker-Gawl . 2 Figs. 21 A-D of the 
present paper show the first plumular leaf of another species, T. Pringlei , 
S. Wats. — a case which is still more striking owing to its greater simplicity. 
In Fig. 21 A the sheath of this leaf is seen enclosing the terminal plumular 
bud (ax.). In Fig. 21 B dorsal invaginations are beginning to make their 
appearance to right and left, while in Fig. 21 C 'the upper limit of the 
sheath has been passed, and the origin of the laminations, by means of 
deep invaginations of the petiole, is clearly displayed. The median bundle 
(: m.b .) is less conspicuous than the main laterals (l. Y and /'.J. In Fig. 21 D 
the ‘plicate’ form is fully attained. The cotyledon sheath (cot. s.), which 
encloses the first plumular leaf, whose history we have been following, is 
not completely indicated except in Fig. 21 D ; in Figs. 21 A, B, c, its inner 
epidermis alone is shown. The curious vascular arrangement in the sheath 
is due to the fact that the cotyledonary strand (cot. v.b.) doubles on itself 
and is thus cut twice . 3 
1 Arber, A. (1921 s ), Figs, i and 2, p. 103. 2 Arber, A. (1921 1 ). 
3 Sargant, E., and Arber, A. (1915), p. 168 and Text-fig. 8. 
